Assad is coming under increasing international pressure due to his brutal crackdown on anti-government protesters. On Wednesday at least 18 were killed when Assad's forces shelled residential neighborhoods in the now-encircled city of Homs.

The confidential report, obtained by Reuters, said the majority of Iran's embargo breaches have been arms sales to Syria. Western diplomats say Syria passed the arms on to Hizbullah and Arab terrorist groups in Israel.

The report by the UN Security Council's "Panel of Experts," which reports on compliance with four rounds of UN sanctions imposed on Iran for refusing to halt its nuclear enrichment program, also asserts Tehran flouts the sanctions as it continues to develop its atomic program.

Diplomats said Russia offered a procedural justification for objecting to the publication of the report -- that the Security Council's Iran sanctions committee must discuss the report before disclosure to the public.

"Eventually they'll have to give in but we don't know how long it will take," a diplomat said.

Russia has been able to stonewall the Iran report because decisions about Security Council reports are made by consensus.

Russia's move comes as British and French diplomats attempt to revive plans for the Security Council to condemn Syria for its crackdown against demonstrators.

A previous attempt failed after Russia, China and India objected to the proposed condemnation.

China has prevented the publication of similar expert panel reports on North Korea and Sudan, two countries that Beijing routinely tries to shield from Security Council criticism, for as long as half a year.

Russia has long acted as Iran's protector on the council, working hard to dilute the four sanctions resolutions on Iran between 2006 and 2010 before voting in favor of them. Russia has close commercial ties to the Islamic Republic.

Iran's Russian-built Bushehr nuclear power plant, activated Tuesday, is expected to be fully operational within weeks.